I have been contacted regarding making a groom's cake for a wedding that is expecting 75 guests.

Generally how many servings should the grooms cake consist of when there is going to be a wedding cake as well?

I would talk to the bride about this. Usually the groom's cake is a different flavor. So if she'd like to cut servings from her cake in order to serve this one to guests as well, that can be done. Or the grooms cake can just feed him for later on, like on the honeymoon. I've seen it done both ways: an equal share between cakes, and mostly bride's cake with just a small groom's cake. I think it's up to what the couple wants, really.

there is no generally. if someone is ordering a grooms cake, then like any other cake they need to tell you what kind of cake, what flavor, and how many servings they want. and make sure they know what YOUR serving size is so they can order accordingly.